Steam turbine control system



Dec. 29, 1970 i TETSUZO SAKAMOTO ET AL 3,551,066

STEAM TURBINE CONTROL SYSTEM 3 Sheets-Sheet 1 Filed Oct. 18. 1968 Dear. 29, 1970 TETSUZQ SAKAMOTO ET AL. 3,551,066

STEAM TURBINE CONTROL SYSTEM Filed Oct. 18, 1968 3 Sheets-Sheet 2 F56. 2(0) FIG. 2(b) Dec. 29, 1970 'rg'rsuzo SAKAMQTO ET AL 3,551,065

STEAM TURBINE CONTROL SYSTEM Filed Oct. 18, 1968 3 Sheets-Sheet 5 United States Patent 3,551,066 STEAM TURBINE CONTROL SYSTEM Tetsuzo Sakamoto and Osamu Ninomiya, Yokohamashi, Japan, assignors to Tokyo Shibaura Denki Kabushiki Kaisha, Kawasaki-shi, Kanagawa-ken, Japan, a joint-stock company of Japan Filed Oct. 18, 1968, Ser. No. 768,832 Claims priority, application Japan, July 15, 1968, 43/ 19,028 Int. Cl. F03b 15/08 U.S. Cl. 415-40 Claims ABSTRACT OF THE DISCLOSURE Either of a full-arc mode cam and a partial-arc mode cam of a cam pair corresponding to each of several control valves of respective inlet nozzles of a steam turbine is selectively caused to control the corresponding control valve thereby to control the flowrate of steam supplied to the nozzle. Changing over from one mode (full-arc or partial-arc) of operation can be carried out readily and smoothly during the operation of the turbine.

BACKGROUND OF THE INVENTION This invention relates to techniques for controlling vapour-driven turbine engines (hereinafter referred to as steam turbines) and more particularly to a new steam-turbine control system having a mechanism capable of changing over or switching from one to the other between full-arc running and partial-arc running operations.

In general, the nozzles of a steam turbine are disposed in front (upstream) of the first-stage turbine blades or buckets and discharge steam through a plurality of openings in arcuate parts spaced in the circumferential direction of a circle coaxial with the turbine. The term fullarc running as used herein refers to that due to a control system whereby the steam supply rates to these plurality of separate nozzles are controlled simultaneously in unison. The term partial-arc running as used herein refers to that due to a control system whereby the steam flowrate for these nozzles is controlled successively for each nozzle in accordance with a predetermined schedule with the object of obtaining the maximum number of nozzles to which steam is supplied at the most efficient rates.

Accordingly, partial-arc control has the advantage of higher efiiciency than full-arc control during partial load operation. Partial-arc control, however, is disadvantageous in the case when the total steam supply rate undergoes an abrupt variation such as that at the starting of the turbine or that due to an abrupt fluctuation in the turbine load since such a variation causes an abrupt variation in the steam supply rate to one part of the turbine, whereby large thermal stresses develop in parts such as the turbine casing.

On the other hand, since the supply of steam to the turbine under full-arc control is accomplished through the nozzles around the entire periphery of their arrangement circle, irregular heating of the various parts of the turbine does not occur. Therefore, full-arc control is superior to partial-arc control in the above described case of abrupt variation in the steam supply.

That is, while partial-arc control produces high elliciency, it has the disadvantageous tendency to give rise to larger thermal stresses in the turbine than full-arc control. Accordingly, a system capable of effecting both full-arc operation and partial-arc operation is highly desirable.

One example of such a system is described in U.S. Pat. 3,027,137. According to this patent, each of the nozzles is provided with a control valve for controlling the steam supply rate to the corresponding nozzle, and

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all of these control valves are maintained fully open during the starting of the turbine, which starting is accomplished by gradually opening a stop valve provided upstream from the control valves to control together the steam supply to all control valves and normally operating in its fully closed or fully opened state.

On the other hand, during normal running operation of the turbine, the above mentioned stop valve is left fully open, and the control valves of the nozzles are controlled separately in sequence. That is, at the time of full-arc operation, steam supply control is carried out by means of the stop valve and not the control valves, while at the time of partial-arc operation, control is carried out by means of the control valves.

While the above described prior system has many merits, it is not completely satisfactory principally because of the following shortcoming. The system, which must efiect control in response to a speed governor, include two sub-systems, i.e., one comprising the step valve and the other comprising the control valve group. This means that the control mechanism necessarily becomes extremely complicated. Furthermore, since full-arc control is carried out by a stop valve, the range within which full-arc control can be accomplished is narrow.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steam turbine control system which is capable of performing full-arc control and partial-arc control, and in which the above described ditficulties of the prior system are overcome by controlling only the control valves.

Another object of the invention is to provide a control system in a steam turbine having a simple mechanism by which changing over from full-arc control to partial-arc control and, moreover, changing over from partial-arc control to full-arc control can be freely carried out during running operation of the turbine.

Further objects of the invention will become readily apparent from the following description.

According to the present invention, briefly summarised, there is provided, in a steam turbine of the aforementioned class, a control system characterised by the combination with the turbine of first and second groups of cam means corresponding respectively to control valves for respective turbine nozzles and operating in response to movements of a speed governor to effect, respectively, full-arc and partial-arc control of the control valves and selection means for selectively changing over the mode of control between full-arc and partial-arc.

The nature, principle, details, and utility of the in vention will be more clearly apparent from the following detailed description beginning with a general consideration and concluding with a description of an example of preferred embodiment of the invention, when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a schematic diagram, partly in section and partly in perspective, indicating the general and essential organisation of one example of a control system embodying the invention;

FIGS. 2(a) and 2(b) are longitudinal section showing a lock valve suitable for using in lock means in the system shown in FIG. 1; and

FIGS. 3(a) and 3(b) are longitudinal views showing stretchable relief means for affording relief when a tension exceeding the expected maximum is imparted to certain parts of the system illustrated in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION In the turbine control system according to a preferred embodiment of the invention, there is provided a cam shaft which rotates in accordance with the actuation of a speed governor, and which supports pairs of cams fixed thereto, one pair of cams being provided for each of the control valves of a steam turbine. One of the cams in each pair is for partial-arc control, while the other cam is for full-arc control. Each of the cams actuates a cam lever through a cam follower secured thereto.

Accordingly, of the plurality of cam levers, all of those actuated by cams for full-arc control undergo simultaneous movement in unison, while those actuated by cams for partial-arc control undergo movement successively in sequence.

Thus, one pair of a cam lever for full-arc control and a cam lever for partial-arc control corresponds to each control valve. These cam levers are pivoted at their ends on one side and are coupled at their other distal ends to respective opposite ends of a rockable floating lever through connecting rods each having means for permitting elongation thereof when the rod is subjected to tensile force exceeding a predetermined value.

Movement of approximately the midpoint of the floating lever due to the operation of a cam lever is transmitted to the spool of a pilot valve, which controls a hydraulic servomotor. The pilot valve and servomotor constitute essential components of a mechanism for controlling the corresponding control valve in accordance with the movement of the floating lever.

Each of the opposite ends of each floating lever is provided with locking means whereby either of the ends can be selectively held as a stationary pivot point. Accordingly, when the lever and through which movement of the cam lever for full-arc control is transmitted is held stationary, for example, the floating lever to actuated by movement of the cam lever for partial-arc control, whereby the corresponding control valve undergoes partial arc control. Conversely, when the lever end on the side for transmitting movement of the partial-arc control cam lever is held stationary, the turbine operates under fullarc control.

Furthermore, in changing over from full-arc control to partial-arc control or vice versa during the operation of the turbine, the above described locking means are operated gradually in a balanced manner so as to cause temporary variations in the total flowrate of the sream supplied to the turbine to be within a range of negligible values.

In a specific example of preferred embodiment of the invention as illustrated in FIG. 1, each of a plurality of control valves 10 (only one shown) connected through a suitable pipe to a respective turbine nozzle (not shown) to control the steam supply rate thereto is actuated by a hydraulic servomotor 12 controllably operated by a pilot valve 13.

The pilot valve 13 has ports 13b and 130 connected respectively through suitable piping to the upper and lower (as viewed in FIG. 1) ports 12b and 120 of the servomotor 12. The pilot valve 13 is further provided with a hydraulic fluid supply port 13d connected through suitable throttling means for regulating the operational speed of the servomotor 12 to a hydraulic fluid source (not shown) and with a drain port 13e.

The actuation movement of the piston 12a of the servomotor 12 is ordinarily transmitted to a valve body 11 of the control valve 10 by a lever 14 which is pivoted at its left (as viewed in FIG. 1) end. This left end of the lever 14 is coupled to a compensation linkage including a link rod 15, a lever 16 pivoted on the control valve 10, and a rod 17 pivoted at one end thereof on the cylinder of the servomotor 12 and is so positioned that errors in the control of the degree of opening of the control valve 10 due to variations in the relative positions of the control valve body 11 and the servomotor 12 are compensated for.

A linkage 18, which is a restoring mechanism of known type, is connected between the piston 12a of the servomotor 12 and the valve stem 13a of the pilot valve 13. The

outer (upper as viewed in FIG. 1) end of a piston stem of a hydraulic relay 19 is coupled to an intermediate part of this restoring linkage 18, the piston stem being biased toward the interior of the cylinder of the relay 19 by a spring. During normal operation, the relay 19 is caused to be in the state shown in FIG. 1 by hydraulic fluid supplied through a port in the bottom of relay cylinder.

At the time of an emergency, that is, when the tur- 'bine speed rises abnormally, the hydraulic fluid in the lower part of the cylinder is drawn out to cause the piston stem of the relay 19 to descend abruptly and thereby to cause the control valve 10 to close rapidly. Impact loads produced at the time of 'such emergency stopping are absorbed by a spring damper 20.

While the foregoing description and following descrip tion relate to the mechanism for operating a single control valve 10 for the sake of simplicity of description, it is to be understood, of course, that a control valve is provided for each of a plurality of turbine nozzles, and each of the control valves is controlled by the same mechanism as described above. That is, if there are four nozzles, for example, four mechanisms, each as descirbed above are provided; if there are six nozzles, six like mechanisms are installed.

The control valve 10 is controlled by the straight-line movements of a rack 21 in response to signals from a speed governor (not shown), these movements being transmitted through a mechanism of the following description to the above described mechanism. The rack 21 is meshed with a pinion 22 fixed to one end of a cam shaft 23 and, in response to variations in the turbine speed due to causes such as fluctuations in the turbine load, rotates the cam shaft 23.

The cam shaft 23 supports pairs of cams fixed thereto, each pair corresponding to one of the turbine nozzles and consisting of a cam for full-arc control and a cam for partial-arc control. In FIG. 1 a first cam pair 24 of a partial-arc control cam 24p and a full-arc control cam 24f and a second cam pair 25 of a partial-arc control cam 25p and a full-arc control cam 25 are illustrated. The other cam pairs (not shown) are successively provided on the extension of the cam shaft 23. All of the full-arc control cams have the same shape and are fixed to the cam shaft 23 with the same phase relationship. On the other hand, the partial-arc control cams do not have the same shape since they are adapted to control the seqential opening and closing of the control valves in accordance with a predetermined schedule.

The intercoupling of the above described cams with their respective control valves will now be described with respect to a single cam pair (the second cam pair 25) for the sake of simplicity.

The cams 25p and 25 can be contacted and followed by respective cam followers fixed respectively to intermediate parts of cam levers 26p and 26 pivoted at their ends on one side on pivots 27p and 27 The cam levers 26p and 26 at intermediate parts thereof are coupled respectively through rods 29p and 29 to pistons 2812a and 28fa of hydraulic motor cylinders 28p and- 28 whereby the cam followers of the cam levers 26p and 26 can be caused to engage with confronting cams 25p and 25f selectively whenever necessary. That is, cylinders 28p and 28 have, at their piston-rod '(upper) ends, ports are connected by way of suitable piping to ports 30d and 30:: of a changeover valve 30 operable by an electromagnet or a solenoid 3% whereby pressurized hydraulic fluid can be selectively supplied to the cylinders 28p and 28].

The changeover valve 30 has a spool with a stem 30a, which is biased constantly toward the left (as viewed in FIG. 1) by a spring provided between the stem 30a and the valve casing and is'normally in the state indicated in FIG. 1. When the electromagnet (solenoid) 30b is energised, theaspool of the valve 30 moves to the right.

More specifically, hydraulic fluid is supplied from a hydraulic fluid source (not shown) to a fluid supply port 300 of the valve 30 and is further supplied through the port 30d when the electromagnet 30b is deenergized to the motor cylinder 28p thereby to cause the cam follower of the cam lever 26p to engage the cam 25p. When the electromagnet 30b is energized, the hydraulic fluid is supplied through the port 30e to the motor cylinder 28 there by to cause the cam follower of the cam lever 26 to engage cam 25f.

The distal ends of the cam levers 26p and 26 are respectively coupled through rods 32p and 32 and relief springs 33p and 33 to opposite ends 31p and 31 of a floating lever 31. The midpoint 310 of the lever 31 is connected by a link rod 34 to one end of a lever 35 coupled at an intermediate part thereof to the outer end of the valve stem 13a of the aforementioned pilot valve 13. Thus, movements of the midpoint 31c of the lever 31 are transmitted through the rod 34 and lever 35 to the spool stern 13a of the pilot valve 13.

The ends 31p and 31 of the lever 31 are further provided with lock cylinders 36p and 36 coupled respectively thereto by way of respective rods so as to enable these ends to become, selectively, stationary pivotal points. These lock cylinders 36p and 36 are controllably operable by hydraulic fluid controlled by relief valves 37p and 37 1.

As shown in FIG. 2, each lock cylinder (36p or 361) has a large-diameter piston 361 biased downwardly (as viewed in FIG. 2) by a spring and provided with a hollow stern 362 and has a small-diameter piston 363 fixed to a piston rod 364 passing coaxially through the bore of the hollow stem 362. The pistons 36.1 and 363 operate slidably in respective cylinders within a valve casing, which is provided at a point between the two pistons with a port 365.

When hydraulic fluid is not being supplied through the port 365 into the space between the two pistons 361 and 363, the large-diameter piston 361 is engaged with a terminal stepped shoulder at the lower end of its cylinder and is then in its lowermost position, while the piston rod 364 of the small-diameter piston 363 is free to move slidably up and down as indicated in FIG. 2(a).

The piston rod 364 is provided near its outer end with an abutment flange 366. When hydraulic fluid is supplied through the port 365, the large-diameter piston 361 is forced upward, overcoming the biasing force of its spring, to its uppermost position, while the small-diameter piston 363 is forced downward until the flange contacts and engages with the outer end of the stern 362 of the largediameter piston 361 as indicated in FIG. 2(b).

That is, when hydraulic fluid is received through the port 365, the piston rod 364 of the small-diameter piston is locked in approximately the middle position of the stroke through which it can move. Each lock cylinder is so disposed that when it is placed in its locked state, the pertinent end of the lever 31 shown in FIG. 1 is fixed at approximately the midpoint of range of travel of the end of the lever 31 due to the operation of the corresponding cam lever.

The ports 365 of the two lock cylinders 36p and 36 are connected by way of suitable throttling devices 38p and 38f to a hydraulic fluid source (not shown). The aforementioned relief valves 37p and 37 f are connected to the lines between the lock cylinders and the throttling devices to control the supply of hydraulic fluid to the lock cylinders.

Each relief valve (37p or 37 1) comprises a casing cylinder having a fluid discharge port at its upper part and a fluid inlet port at its lower part, a piston biased downwardly (as view in FIG. 1) within the cylinder by a spring and having a central through hole, a valve body capable of fitting into and closing the central hole of the piston, and a valve stem fixed to the valve body and extending downwardly through the bottom of the cylinder.

The other ends of the valve stems of the relief valves 37p and 37 f are provided respectively with racks 39p and and 39 which are meshed with mutually opposite directional relationship, as shown in FIG. 1, with pinions 40p and 40 fixed to a common shaft 41 driven by a motor 42. Since these relief valves can be caused to control the discharge fluid flowrate by varying the force with which their valve bodies are pressed against the central holes of their respective pistons,it is possible by controlling the rotation of the shaft 41 to control the increase and decrease of the fluid pressures within the lock cylinders 36p and 36 with mutually and symmetrically opposite tendencies.

Then, when hydraulic fluid is being supplied to the lock cylinder 36], for example, the lever end 31 is fixed at a position corresponding to approximately the midpoint of the range of movement of the cam lever 26 The cam lever 26 however, may 'be positioned above the above mentioned midpoint in some cases depending on the phase of the cam 25 In such a case, the aforementioned relief spring 33 stretches lengthwisely and makes possible the maintenance of the lever end 31 at the required position.

As shown in FIG. 3, each relief spring 33 has an upper spring retainer 331, a lower spring retainer 332, and a tension coil spring 333 stretched between these spring retainers. A hollow cylindrical part is fixed to and extends downward from the upper spring retainer 331 and at its lower end is normally abutting against the lower spring retainer 332. On the other hand, a column-like rod is fixed to and extends upward from the lower spring retainer 332, this rod being slidably fitted in the cylindrical part of the upper spring retainer 331.

Accordingly, when a compressive force is applied to the spring retainers 331 and 332 in assembled state, the relief spring assembly remains in the state indicated in FIG. 3(a). On the other hand, when a tensile force is applied to the spring retainers 331 and 332, the state indicated in FIG. 3(a) is maintained while the applied force is below a certain value, but when the applied force exceeds this value, the relief spring assembly is stretched, and the distance between the spring retainers 331 and 332 increases as indicated in FIG. 3(b). That is, the relief springs 33p and 331 stretch when subjected to a tensile force exceeding a predetermined value but can be considered to be substantially rigid structures in all other cases.

The turbine control system of the above described organisation according to the invention operates in the following manner.

It will first be assumed that the system is operating in the state indicated in FIG. 1, in which, of course, hydraulic fluid at suitable pressure is being supplied throughout the hydraulic system circuit. At this time, ports 30c and 30d of changeover valve 30 are communicative, whereby hydraulic fluid is being supplied to cylinder 28p to cause the cam follower of camlever 26p to engage cam 25p for partial-arc control. On the other hand, hydraulic pressure is not being supplied to cylinder 28 coupled to cam lever 26; for full-arc control.

In relief valve 37p, the fluid inlet and discharge ports are in communicative state, whereby lock cylinder 36p is not being supplied with pressurised fluid, and end 31p of lever 31 is thereby free to move up or down. In relief valve 37 on the other hand, the passage from the inlet port to the discharge port is almost fully closed, whereby the fluid pressure within lock cylinder 36] is high, and lock cylinder 36 is therefore in a locked state. As a result, one end 31p of lever 31 becomes a free end, while the other end 31 becomes a pivoted stationary end.

Consequently, the movement of cam lever 26p for partial-arc control due to the action of cam 25 is transmitted through rod 32p and relief spring 33p to left end 31p of lever 31. That is, lever 31 moves around its right end 31 as a pivotal point in accordance with the actuation of partial-arc cam 25p. Lever 31 thereby operates through link rod 34 and lever 35 to actuate stem 13a of pilot valve 13, thereby operating control valve body 11 through servomotor 12.

Thus, with the system in the illustrated state, the control valve 10 is being controlled in accordance with partial-arc cam p. That is, the turbine (not shown) is being operated according to a partial-arc mode of operation. It can be readily seen that, in the above described case, relief spring 33p operates constantly as a substantially rigid body, while relief spring 33] with fixed lower end elongates in accordance with necessity.

Next, the operation of starting the turbine by full-arc control will be considered. First, electromagnet b is energised to move the spool of changeover valve 30 to the right (as viewed in FIG. 1), whereby ports 30c and 30e are placed in communicative state to supply hydraulic fluid to cylinder 28 Motor 42 is also started, and relief valve 37p is closed thereby to place lock cylinder 36p in a locked state and to-unlock lock cylinder 36 Thus, the force causing the cam follower of partial-arc cam lever 26p to press against cam 25p is removed, and left end 31p of lever 31 is caused to be stationary. At the same time the cam follower of full-arc cam lever 26 is pressed against cam- 25 and the mechanism is thereby placed in a state whereby lever 31 can be operated in accordance with the angle of rotation of cam 25 Accordingly, when cam 25 is rotated by rack 21 according to a predetermined sequence in the direction for starting the turbine, lever 31 rises according to a full-arc mode of operation, and pilot valve 13 connected to midpoint 310 of lever 31 through lever and link rod 34 is thereby controlled.

More specifically, the spool of pilot valve 13 is raised, whereby ports 13d and 13c are caused to be communicatable to supply hydraulic fluid to lower port 12c of servomotor 12. Piston 12a of servomotor 12 accordingly rises and controllably moves control valve body 11 in the opening direction. When control valve 10 has been opened to a predetermined degree of opening, pilot valve 13 is restored to its neutral state by the operation of restoring linkage 18.

Partial-arc cam' 25p and cam lever 26p are separated while the cam rotational angle is small. The reason for this is that left end 31p of lever 31 is held stationary at a position at approximately the midpoint of its range of movement, and relief spring 33p cannot be compressed. When the cam rotational angle becomes large, however, the cam follower of cam lever 26p engages cam 25p and is raised thereby. In this case, the length of relief spring 33p is elongated and left end 31p of lever 31 continues to be held stationary in the predetermined position. Therefore, the turbine can be operated under full-arc controlled up to full load operation.

Next, the transfer or changeover from full-arc mode of operation to partial-arc mode of operation will be considered. When changeover from full-arc mode to partial-arc mode is carried out with the turbine running under partial load, there is almost no change in the total flowrate of steam supplied to the turbine before and after the changeover, but the flowrates of the steam supplied to the different nozzles, in general, vary. Under full-load operation of the turbine, of course, the flowrates to the different nozzles do not vary.

However, when the flowrates of steam supplied to the different nozzles vary, rapid changeover of modes of operation is not desirable since high thermal stresses are produced in the turbine. Therefore, in accordance with the present invention, this changeover is carried out gradually.

More specifically, in order to cause the cam follower of cam lever 26p for partial-arc control to be pressed against cam 25p, electromagnet 30b of changeover valve 30 is deenergised, and hydraulic fluid is supplied to cylinder 28p. Motor 42 drives shaft 41 in a manner such that valve stem 39p of relief valve 37p descends gradually, and valve stem 39] of relief valve 37f rises gradually. Consequently, the pressure of the fluid within lock cylinder 36p decreases gradually, and, in symmetrically converse relationship to this decrease, the pressure of the fluid within lock cylinder 36] rises gradually. That is, the

8 system undergoes a gentle transition to the state indicated in FIG. 1.

During this transition, the position of midpoint 310 of lever 31 coupled to valve stem 13a of pilot valve 13 undergoes no change whatsoever in some cases but, in general, shifts to cause a variation in the degree of opening of pilot valve 13. This shift of the position of midpoint 310 of lever 31 is the minimum possible when one end of lever 31 is held as a stationary pivotal point since that end is then held at the midpoint position of its range of movement.

Furthermore, the pair of lock cylinders 36p and 36 operate in a substantially symmetrical but mutually reverse manner, one changing from its unlocked state to its locked state and the other changing from locked to unlocked state. Moreover, the speed of this change operation can be controlled as desired by operating and controlling relief valves 37p and 37f.

Thus, excessive variations in the flowrate of steam in jected through the nozzles due to shifting of the position of midpoint 310 of lever 31 can be very readily prevented from adversely affecting the turbine. Furthermore, it will be apparent that such a measure will not obstruct the purport of the invention. The reason for this is that instantaneous transition from the full-arc mode to the partial-arc mode is not required.

While transition from full-arc control to partial arc control can be carried out successively with respect to the individual control valves, it is also possible to carry out this transition simultaneouslywith respect to all control valves. This latter method is advantageous in that excessive variations in the total steam flowrate at the time of transition can be caused to nullify each other.

As is apparent from the foregoing description, the present invention makes possible, during operation of a turbine, changing over as desired from full-arc control to partial-arc control and, conversely, from partial-arc control to full-arc control.

According to the present invention as described above with respect to a preferred embodiment thereof, there is provided a turbine control system in which a partial-arc control cam and a full-arc control cam forming a cam pair for a respective one of the control valves of a turbine are fixed to a single cam shaft, and each control valve is controllable by one cam selected as desired from the corresponding pair of cams, whereby full-arc control and partial-arc control can be accomplished with a simple mechanism.

We claim:

1. A control system for selectively controlling the movement of an output member in two different modes of operation comprising: a movably mounted output member; first driven cam means for generating a first output signal indicative of a first mode of operation; second driven cam means for generating a second output signal indicative of a second mode of operation; movable transmitting means connected between said output member and said first and second cam means operable when in a first working position to effect movement of said output member as a function of said first output signal and operable when in a second working position to effect movement of said output member as a function of said second output signal; and fluid control means for selectively effecting movement of said transmitting means to said first and second working positions in response to a control signal.

2. A control system according to claim 1; wherein said transmitting means includes a pivotally mounted lever, means mounting said lever for movement to both said first working position to undergo pivotal movement about a first pivot point in response to said first output signal and to said second working position to undergo pivotal movement about a second pivot point in response to said second output signal; and wherein said fluid control means comprises first and second fluid motors Cooperative together to move said lever to said first and second working positions in response to fluid pressure differentials applied thereto, and valve means selectively-actuated in response to said control signal for gradually varying the fluid pressure differentials applied to said first and second fluid motors in mutually opposite directions of variation to effect a corresponding gradual movement of said lever between said first and second working positions whereby the mode of operation of said output member is gradually changed.

3. In a turbine control system having at least one control valve operable to control the admission of working fluid to a nozzle group positioned around the circumference of a first turbine stage in either a partial-arc or full-arc mode of operation, the improvement comprising: actuating means including a movable pilot member for actuating said control valve in response to movement of said pilot member; first driven cam means for generating a first output signal indicative of a partial-arc mode of operation; second driven cam means for generating a second output signal indicative of a full-arc mode of operation; movable transmitting means connected between said pilot member and said first and second cam means operable when in a first working position to effect movement of said pilot member as a function of said first output signal and operable when in a second working position to effect movement of said pilot member as a function of said second output signal; and fluid control means for selectively effecting movement of said transmitting means to said first and second working positions in response to a control signal; whereby said control valve supplies working fluid in either a partial-arc or full-arc mode.

4. A turbine control system according to claim 3; wherein said transmitting means includes a pivotally mounted lever, means mounting said lever for movement to both said first working position to undergo pivotal movement about a first pivot point in response to said first output signal and to said second working position to undergo pivotal movement about a second pivot point in response to said second output signal; and wherein said fluid control means comprises first and second fluid motors cooperative together to move said lever to said first and second working positions in response to fluid pressure differentials applied thereto, and valve means selectivelyactuated in response to said control signal for gradually varying the fluid pressure differentials applied to said first and second fluid motors in mutually opposite directions of variation to effect a corresponding gradual movement of said lever between said first and second working positions whereby the mode of operation of said pilot member is gradually changed.

5. A turbine control system according to claim 3; wherein said first and second cam means include a common rotationally driven cam shaft.

6. A turbine control system according to claim 3;

10 wherein said transmitting means includes extendable relief means cooperative with each said cam means for temporarily extending the length of said transmitting means whenever an output signal from the particular cam means not corresponding to a working position of said transmitting means exceeds a predetermined value.

7. In a steam turbine control system having a plurality of steam admission control valves operating in response to a signal from a speed governor, each such valve controlling admission of steam to one of several nozzle groups around the circumference of a first turbine stage, and cam means for the control of steam admission in either a full-arc or partial-arc mode of operation, the improvement which comprises: a single cam shaft provided with a first group of cams of the same phase for controlling the full-arc mode of operation and a second group of cams of different phase for controlling the partial-arc mode of operation; means to rotate said cam shaft in accordance with the operation of a speed governor; cam levers each associated with a different one of said cams of said first and second groups; changeover valve means for causing said cam levers to cooperate with either said first group of cams or said second group of cams; and a plurality of selection means for said control valves; each one of said selection means including a first lever connected to one of said cam levers cooperating with a cam of said first group, a second lever connected to one of said cam levers cooperating with a cam of said second group, a cross lever pivotally connected between said first and second levers for controlling one of said control valves, locking means for selectively locking said first and second levers, and means for selectively operating said locking means to cause said cross lever to pivot about one end thereof connected to either one or the other of said first and second levers.

8. The steam turbine control system according to claim 7; wherein each of said first and second levers include resilient relief means operable to elongate when subjected References Cited UNITED STATES PATENTS 7/1963 Eggenberger et a1. 73 10/1968 Johnson et a1, 415-43X ROBERT M. WALKER, Primary Examiner 

